How to Grow Onion Seed Indoors

Fill a 12-inch pot with potting soil. Use a pot with a drainage hole in the bottom as onions need well-drained soil. Place a saucer or bowl under the pot to catch any excess water.

Water the soil until thoroughly moist. It may take at least 10 minutes for the water to soak through all of the soil, so be patient. You can poke your finger in the pot's drainage hole to determine whether or not the soil is thoroughly moist.

Remove 1/2 inch of soil from the top of the pot. Put the soil in a bowl or cup.

Place an onion seed in the center of the pot. If you plan to transplant the onions later on, make multiple planting sites in the pot, with each seed spaced 3 inches apart. It's best to plant two or three seeds in each spot as some seeds may not germinate.

Sprinkle the soil from the bowl over the seeds. Spread the soil over the seeds to ensure the surface is flat. Do not pack the soil down, however, as it needs to be loose for the onion seedlings to sprout.

Place the pot in a sunny window, preferably a south-facing window. Onions do best with constant sun exposure throughout the day, but this may not be possible in all homes. Choose an east-facing window if you do not have one that faces south.

Monitor the moisture level of the soil every day. Add enough water to moisten the soil each time it dries out.

Follow the same watering regimen until you're ready to transplant the onions outdoors. If you want to keep the onion in the pot for the entire growth cycle, follow the original watering regimen until harvest season.